This invention relates to heating apparatus for treating a web of material, and, more particularly, to an infrared (IR) heater for heating a paper web.
Conventional papermaking machinery for producing a continuous sheet of paper includes equipment to set the sheet properties of the paper as it is being manufactured. One of the more basic operations on a paper machine is control of the cross-direction moisture profile by drying with gas infrared heaters. Such heaters are also useful for drying coatings onto a paper web.
Typical infrared heating systems designed specifically for papermaking machinery comprise a series of independently controllable heater units or emitters that are positioned over the paper web in the cross-machine direction CD. Each heater unit consists of a porous refractory ceramic matrix that is fitted into a metallic housing. A plurality of housings are positioned side by side to extend across the web. The porous ceramic matrix is bonded to an aluminum housing with silicone to define a plenum chamber. The plenum chamber of the housing is supplied with an air/fuel mixture via an inlet that connects to a fuel supply. Gaseous fuel in the form of natural gas or propane is mixed with air in a 1:10 ratio to create the air/fuel mixture. Combustion occurs only on the outer 1 mm of the ceramic matrix to provide fast heat up times of about 5 seconds and fast cool down times of about one second. This behavior is essentially due to the ability of the infrared emitting particles incorporated in the matrix to radiate the heat generated, thus preventing the combustion flames.from destroying the matrix by melting. However, if the emitting particles cannot radiate the heat energy, for example in a crack in the matrix or at the edges between the matrix and the housing, the flame can melt the matrix and propagate due to the open porosity of the matrix. Any such propagating flame will eventually reaching the plenum chamber of the unit creating an explosion. Applicant""s co-pending U.S. patent application Ser. No. 09/638,731 filed Aug. 14, 2000 entitled INFRARED HEATER WITH IMPROVED MATRIX discloses an improved crack resistant matrix, the disclosure of which is incorporated herein by reference.
During normal operation, the temperature of the heater will be about 40xc2x0 C. at the inner surface of the ceramic matrix to between 575xc2x0 C. to 950xc2x0 C. at the exterior surface of the matrix adjacent where combustion occurs. Due to the high thermal output of the heater unit, the ambient temperature can exceed 150xc2x0 C. At this temperature, thermal expansion of the metal housing having a typical wall length of 400 mm can result in over 1.5 mm expansion of the housing with respect to the ceramic matrix. While the exterior matrix surface operates at much higher temperatures, the overall matrix tends to stay relatively cool due to its low thermal conductivity and flame quenching characteristics. The ceramic matrix tends not to expand and may even contract after prolonged use. Expansion of the housing and non-expansion or even contraction of the ceramic matrix tends to stretch and fray the edges of the ceramic matrix material. Repeated heating and cooling results in progressive fraying of the edges of the matrix where it is attached to the frame to create tiny cavities. When the exterior surface combustion flame enters a cavity, it can ignite the gas in the housing resulting in an explosion. Also, the frayed edges of the matrix are more porous and are prone to initiation of flame propagation into the matrix as well as thermal damage to the silicone bond layer as described below.
In addition, in prior art heating unit designs, the ceramic matrix is bonded to the metal housing using an elastomer such as silicone glue. Silicone glue does not seal the ceramic matrix, and it is possible for the gas/air mixture to leak at the side walls adjacent the glue. A cold wall of the housing can arrest the flame propagation from the surface if the wall is in close proximity to the flame front. If the silicone layer is sufficiently thick (xcx9c1 mm), the housing wall tends not to quench the propagating flame front by heat extraction and the flame can enter the plenum chamber igniting the fuel air mixture. Thus, the combustion flame on the external surface of the matrix can propagate to the elastomer bond region and burn, exposing new surfaces on the side wall of the ceramic matrix with access to the gas/air mixture. This leads to disintegration of the bond between the ceramic matrix and the metal housing to allow the flame to ignite the gas in the housing resulting in the same type of explosion described above.
There is therefore a need to develop an infrared burner unit that avoids the thermal expansion/contraction degradation of the ceramic matrix over time and the flame propagation problems of the prior art.
To address the foregoing problems, applicant has developed a new infrared heater, which is constructed so as to minimize the effects of heat expansion of the metal housing.
In one embodiment, this involves providing a flexible sealing member about the perimeter of the ceramic matrix to join the matrix to the housing that is capable of tolerating the differential expansion. This embodiment finds particular application when the housing operating temperature is high ( greater than 150 C). Such a heating unit comprises:
a housing;
a heat resistant, porous ceramic matrix received in the housing having an inner surface, side walls and an external surface, the inner surface of the ceramic matrix and the housing cooperating to define a chamber;
at least one resilient anchoring strip extending between the ceramic matrix and the housing about the perimeter of the ceramic matrix to retain the ceramic matrix in place within the housing and seal the edges of the ceramic matrix; and
an inlet to the housing to admit a gas/air combustion mixture to the chamber whereby the gas/air mixture passes through the ceramic matrix to burn adjacent the external surface of the ceramic matrix to heat the external surface with the resilient anchoring strip being adapted to expand or contract to accommodate relative movement of the ceramic matrix with respect to the housing due to heating.
Preferably, the side walls of the porous ceramic matrix adjacent the at least one resilient anchoring strip are blocked with a blocking agent to prevent flow of the gas/air combustion mixture through the side walls. This ensures that flames on the external surface of the matrix are not able to migrate about the side walls of the matrix to damage or consume the resilient anchoring strip.
In another embodiment, the resilient anchoring strip is not used and the edge of the ceramic matrix is blocked with a blocking agent to prevent flame propagation from the external surface of the matrix to the side walls. In this arrangement, the heating unit comprises:
a housing;
a heat resistant, porous ceramic matrix received in the housing having an inner surface, side walls and an external surface;
a blocking layer to block the side walls of the porous ceramic matrix to prevent flow of a gas/air combustion mixture through the side walls;
an adhesive layer between the housing and the side walls of the ceramic matrix to retain the matrix within the housing, the inner surface of the ceramic matrix and the housing cooperating to define a chamber; and
an inlet to the housing to admit the gas/air combustion mixture to the chamber whereby the gas/air mixture passes through the ceramic matrix to burn adjacent the external surface of the ceramic matrix to heat the external surface.
This heating unit is most useful when the housing temperature is maintained below 180 C and preferably below 150 C by using auxiliary air flow cooling or by incorporation of an insulating medium covering the free edges of the housing which gather most of the reflected infrared radiation. Preferably, the insulating medium is a heat resistant insulating layer provided between the ceramic matrix and the housing to limit heat availability to the housing and thereby thermal expansion of the housing.
In a still further embodiment, the problem of differential thermal expansion of the housing and the ceramic matrix is substantially avoided by eliminating the housing surrounding the ceramic matrix. Instead, the sealed side walls of the ceramic matrix are exposed well above the housing side walls. In this arrangement, the heating unit comprises:
a lower housing having an inlet to admit a gas/air combustion mixture;
a heat resistant, upper porous ceramic matrix having an inner surface, an external surface and blocked side walls, the ceramic matrix being mounted atop the lower housing such that the blocked side walls of the matrix are exposed and the inner surface of the ceramic matrix and the housing cooperate to define a chamber to receive the gas/air combustion mixture;
whereby the gas/air mixture passes through the ceramic matrix to burn adjacent the external surface of the ceramic matrix to heat the external surface, and the blocked side walls prevent flow of the gas/air combustion mixture through the side walls.